Imaging devices, including charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) imagers, are commonly used in photo-imaging applications.
A CMOS imager circuit includes a focal plane array of pixels, each of the pixels including a photosensor, for example, a photogate, photoconductor or a photodiode for accumulating photo-generated charge a portion of the substrate. Each pixel has a charge storage region, formed on or in the substrate, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some imager circuits, each pixel may include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference.
In a CMOS imager, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region accompanied by charge amplification; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing a reset level and pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
CMOS imagers of the type discussed above are generally known as discussed, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524 and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc., which are hereby incorporated by reference in their entirety.
A typical four transistor (4T) CMOS image pixel 10 is shown in FIG. 1. The pixel 10 includes a photosensor 12 (e.g., photodiode, photogate, etc.), transfer transistor 14, floating diffusion region FD, reset transistor 16, source follower transistor 18 and row select transistor 20. The photosensor 12 is connected to the floating diffusion region FD by the transfer transistor 14 when the transfer transistor 14 is activated by a transfer gate control signal TX.
The reset transistor 16 is connected between the floating diffusion region FD and an array pixel supply voltage Vaa_pix. A reset control signal RST is used to activate the reset transistor 16, which resets the floating diffusion region FD to the array pixel supply voltage Vaa_pix level.
The source follower transistor 18 has its gate connected to the floating diffusion region FD and is connected between the array pixel supply voltage Vaa_pix and the row select transistor 20. The source follower transistor 18 converts the charge stored at the floating diffusion region FD into electrical output voltage signals Vrst, which is produced when the floating diffusion region FD is reset, and Vsig, which is produced after charge is transferred by transistor 14 from the photosensor 12 to the floating diffusion region FD. The row select transistor 20 is controllable by a row select signal SEL for selectively connecting the source follower transistor 18 and its output voltage signal Vout to a column line 22 of a pixel array.
An important performance characteristic of any imager is its dynamic range. A large dynamic range is desirable in applications for sensing low light signals and capturing images with large variations in illuminance or brightness. In particular, the dynamic range of an imager can be defined as the ratio of the minimum illuminance the imager detects under saturation to the illuminance the imager detects at a signal-to-noise ratio (SNR) equal to one. The dynamic range of a scene can also be expressed as the ratio of its highest illumination level to its lowest illumination level.
Intrascene dynamic range refers to the range of incident signals that can be accommodated by an imager in a single frame of image data. Examples of scenes that generate high dynamic range incident signals include an indoor room with outdoor window, outdoor mixed shadow and bright sunshine, night time scenes combining artificial lighting and shadows, and in an automotive context, an automobile entering or about to leave a tunnel or shadowed area on a bright day.
FIG. 2 illustrates a block diagram of a CMOS imager device 308 having a pixel array 240 with each pixel being constructed as described above or in accordance with other known pixel architectures. Pixel array 240 comprises a plurality of pixels arranged in a predetermined number of columns and rows. The pixels of each row in array 240 are all turned on at the same time by a row select line, and the pixels of each column are selectively output by respective column select lines. A plurality of row and column lines are provided for the entire array 240. The row lines are selectively activated by the row driver 245 in response to row address decoder 255 and the column select lines are selectively activated by the column driver 260 in response to column address decoder 270. Thus, a row and column address is provided for each pixel.
The imager 308 is operated by a control circuit 250, which controls address decoders 255, 270 for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry 245, 260 which apply driving voltages to the drive transistors of the selected row and column lines. The pixel column signals, which typically include a pixel reset signal Vrst and a pixel image signal Vsig are read by sample and hold circuitry 261 associated with the column driver 260. A differential signal Vrst−Vsig is produced and amplified by a differential amplifier 262. The differential signal is digitized by an analog-to-digital converter 275. The analog-to-digital converter 275 converts the analog differential signals to digital signals that are fed to an image processor 280 to form and output a digital image.
The imager 308 is operated by a control circuit 250, which controls address decoders 255, 270 for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry 245, 260 which apply driving voltages to the drive transistors of the selected row and column lines. The pixel column signals, which typically include a pixel reset signal Vrst and a pixel image signal Vsig are read by sample and hold circuitry 261 associated with the column device 260. A differential signal Vrst−Vsig is produced and amplified by a differential amplifier 262. The differential signal is digitized by an analog-to-digital converter 275. The analog-to-digital converter 275 converts the analog differential signals to digital signals that are fed to an image processor 280 to form and output a digital image.
The illumination-voltage profile of the conventional pixel 10 (FIG. 1) for the photo-charge converted signal Vsig is typically linear as shown in FIG. 3, which illustrates an illumination versus voltage graph of the prior art pixel 10. The pixel's 10 maximum voltage Vmax may be reached at a relatively low level of illumination Imax, which causes the pixel 10 to be easily saturated. When the light captured and converted into charge by the photosensor 12 is greater than the capacity of the photosensor 12, excess charge may overflow and be transferred into the substrate and to adjacent pixels, which is undesirable. Thus, there is a desire and need for an improved saturation response in imager pixels 10 and imagers 308 in general. Improved saturation response will improve the imager's dynamic range.